Email spam, also known as unsolicited bulk email, or junk mail, has become a problem soon after the general public started using the Internet in the mid-1990s. Unsolicited messaging is not limited to email. Examples of other types of spam are: instant messaging spam, Usenet newsgroup spam, web search engine spam, online classified ads spam, mobile phone messaging spam, internee forum spam, etc.
In some instances, providers of email services allow users to report the receipt of spam messages. Based on a spam report received from a user, a representative of the email service provider investigates the content of the reported spam message to determine if the message is indeed spam or is simply offensive to the particular user. If the reported message is determined to be spam, the email service provider may choose to block future messages from the sender of the spam message (also known as a “spammer”).
Because a large portion of the reported messages turns out not to be spam, human review of reported messages can be very wasteful of man-hours. In addition, the human review of reported spam messages tends to be very slow, and in the time that a person analyzes a reported message to determine if it is junk mail, the spammer may inundate an email service (or the Inboxes of the users of the email service) with thousands of unsolicited messages.